The present invention relates to a method for molding an article by using ceramic or glass or metal powders, and more specifically, to a method for curing a highly concentrated slurry of the powders in a dispersed state by a dispersing agent by adding a specific reactive substance to the slurry to destroy the dispersed state.
In order to mold an article from materials such as ceramic or metal powders, or mold cermet materials, and inclined functional materials in which ceramic and metal powders are mixed, compression molding, extrusion, injection molding, casting and the like have been used to form various high-strength materials which excel in heat resistance and wear resistance.
There is no need to say that those forming methods have various advantages. On the other hand, however, compression molding, for example, is limited to forming simple shapes such as discs or prisms, and extrusion is limited to forming products having a constant profile in the cross-section vertical to the extruding direction, such as rods and honey-combs. In order to form products having complicated shapes using such methods, the molded green body must be calcined into an article having a suitable strength, processed mechanically, and then finally sintered.
In injection molding, since large quantities of a resin and a binder must be added to the material powder, the density of the molded articles is low, the degreasing process takes a long time, and sintering shrinkage is large. Therefore, the use of injection molding is limited to relatively small products having complicated shapes.
Furthermore, casting has problems in that the mechanical strength of the molded articles is low. Moreover, the production of a plurality of products having a thickness of a certain level is difficult due to the fluctuation in the conditions of slurry to be controlled in the case of the slurry removal method; and that the internal defects such as shrinkage cavities are easily produced in the case of the solid method. In either method, the dispersed state of the slurry must be maintained always at a predetermined, preferable state.
In order to solve such problems in conventional methods, methods for forming an article from powders, which facilitate forming and processing into complicated shapes, and producing uniform products with small sintering shrinkage, have been investigated. The near-net method in which a slurry is produced by dispersing a large quantity of powders and then curing the slurry is one of the solutions. According to the near-net method, a product having a complicated shape can be produced by the use of a mold having the intended shape. The uniformly dispersed state of the slurry results in a dense packing of powders in the molded articles.
As one of such near-net methods, the inventors of the present invention have developed a method for obtaining elastic molded articles by curing water-based slurry containing a large quantity of powders by adding various water-absorbing substances to the slurry as disclosed in the Japanese Patent Application Laid-Open No. 8-127087. Also, in Japanese Patent Publication No. 5-48724, a method is disclosed for forming by freezing the dispersing agent in the slurry, and in Japanese Patent Publication No. 7-22931, a method is disclosed for obtaining molded articles by adding a resin and its curing agent to the slurry.
However, in the molding method from powders using the water-absorbing substances or the resin and its curing agent, there is limitation in increasing the density of the molded articles because resins and the like, which are not required in the final products, are added to the slurry. Also, since the viscosity of the slurry increases sharply by the addition of resins and the like, the workability of molding is worsened, and studies for further improvement are required. Furthermore, in the method in which the dispersing agent in the slurry is frozen, the temperature for forming and of the molded articles must be controlled strictly, and when the molded articles are processed before freeze-drying, they must be processed under a temperature condition at which the molded articles are solid.